1. Field of Invention
The invention relates to a method for data write-in for a toggle magnetic tunnel junction (MTJ) element, and in particular to a data write-in control circuit for accessing magnetic random access memory by way of self-reference
2. Related Art
The magnetic random access memory (MRAM) is a kind of non-volatile memory, and is used to store and record the data by making use of its electric resistance characteristics, thus having the advantages of non-volatility, high density, high read/write speed, and radiation resistant, etc. The major memory unit of Magnetic Random Access Memory (MRAM) is a magnetic memory unit produced between a write bit line and a write word line, and it is of a stack structure made of multi-layer magnetic metal material, thus it is also referred to as a Magnetic Tunnel Junction (MTJ) element, having a stack structure formed by stacking a soft magnetic layer, a tunnel barrier layer, a hard magnetic layer, and a non-magnetic conduction layer in sequence.
The toggle magnetic tunnel junction (MTJ) element, having the advantages of wide operation range and high heat stability, and thus is well suitable for application in an embedded system.
The memory state of “0” or “1” of MJT element is determined through the parallel or anti-parallel alignment of the magnetic-moment configurations of two layers of ferromagnetic material adjacent to the tunnel barrier layer. As such, the data-write-in is realized through a cross selection of a write bit line and a write word line, wherein, the change of magnetization direction of a memory layer magnetic material is achieved through a magnetic field generated by the current flowing in the write bit line and write word line, so as to change the value of electric resistance, hereby realizing the objective of data-write-in.
Referring to FIG. 1 for a schematic diagram of a structure of an exemplary toggle magnetic tunneling junction (MTJ) element. As shown in FIG. 1, the MTJ element is composed of a anti-ferromagnetic layer 10, a pinned layer 20 formed on anti-ferromagnetic layer 10, a tunnel barrier layer 30 formed on pinned layer 20, and a free layer 40 formed on top of tunnel barrier layer 30. The pinned layer 20 and free layer 40 are both of synthetic anti-ferromagnetic configurations. An upper electrode 51 is formed on top of the freedom layer 40, while a lower electrode 52 is formed below an anti-ferromagnetic layer 10. The upper electrode 51 and lower electrode 52 are connected with a metallic wire, thus forming a route for reading data. Located above and below the upper electrode 51 and the lower electrode 52 are a write bit line (WBL) and write word line (WWL) respectively, as shown in FIG. 2, so that a magnetic field is generated, when a write-in current flows through. In addition, the upper electrode 51 is connected to a read-bit-line (RBL).
The anti-ferromagnetic layer 10 is made of an anti-ferromagnetic material, such as PtMn or IrMn. The pinned layer 20 formed on an anti-ferromagnetic layer 10 is a stack formed by more than one ferromagnetic layers. As shown in FIG. 1, the composite anti-ferromagnetic pinned layer is a three-layer structure formed by stacking ferromagnetic material, non-magnetic metal, and ferromagnetic material sequentially, so that the directions of magnetic moments of the two ferromagnetic layers are in anti-parallel alignment, and it can be made by for example, CoFe/Ru/CoFe, NiFe/Ru/NiFe, or CoFeB/Ru/CoFeB. The tunnel barrier layer 30 formed on pinned layer 20 is made of a material, such as AlOx or MgO. The free layer 40 formed on tunnel barrier layer 30 is a stack of more than one layer of ferromagnetic material, and it can be selected from one of NiFe, CoFe, CoFeB.
In FIG. 1, the pinned layer 20 is a three-layer structure, composed of magnetic layers 21 and 23 made of ferromagnetic material, and a middle layer 22 made of non-magnetic metal. In addition, the free layer 40 is also a three-layer structure, composed of magnetic layers 41 and 43 made of ferromagnetic material, and a middle layer 42 made of non-magnetic metal. The magnetic layers 41 and 43 in free layer 40 each having its respective magnetic moments 61 and 62, and are kept in anti-parallel alignment through coupling of the middle layer 42. The magnetic moments 63 and 64 of the magnetic layers 21 and 23 in pinned layer 20 are kept in anti-parallel alignment. The directions of magnetic moments of magnetic layers 41 and 43 in free layer 40 can rotate freely through applying a magnetic field; while the magnetization directions of magnetic layers 21 and 23 in pinned layer 20 will not rotate through applying a magnetic field, thus serving as a reference layer.
In writing data into memory, the method usually utilized is first selecting a memory unit through the intersection of the induced magnetic fields generated by a write bit line and a write word line, and then changing its value of electric resistance through varying the magnetization direction of the free layer 40. While reading data from memory, current must be supplied to the magnetic memory unit thus selected, and then reading the value of electric resistance in determining the digital value of the data.
Due to the anti-parallel coupling effect between the magnetic layers 41 and 43 of the free layer 40, such that the write-in operation area and sequential introduction write-in manner of toggle MTJ element are as shown in FIGS. 3A & 3B, and is referred to as a first-in first-out mode, namely, the current that is made to conduct and flow first, will be made to stop first. For example, in FIG. 3A, the current IW on a write word line is made to conduct and flow first, then the current IB on a write bit line is made to conduct and flow. Thus, only when the current IW on write word line is made to stop, then the current IB on the write bit line will be made to stop. Conversely, the write operation areas 71 and 73 are as shown in FIG. 3, when the current IW of write word line is first made to conduct and flow, then the magnetic moments 61 and 62 of magnetic layers 41 and 43 will rotate in a clockwise direction 72; and when current IB of write bit line is made to conduct and flow, then the magnetic moments 61 and 62 of magnetic layers 41 and 43 will rotate in a counter-clockwise direction 74.
The rotation of the magnetic moment is unidirectional circulation because of writing waveform for the toggle magnetic memory. Prior to data writing, the stored data is accessed to be compared with the data to be written into the memory. Whether the operation of the data writing is then performed according to the comparison result. This process is called Read-Before-Write (RBW). Therefore, such process decreases the writing speed.
Access by way of self reference is disclosed to improve the writing speed. The memory unit itself is used for self reference. The process involves with recording the voltage or the current in an initial state. Then, the data “0” or “1” is then written into the memory. The voltage or the current in an initial state and that after the data is written is compared. The compared result is then used to determine the data initially stored in the memory. However, the initial data stored in the memory may be changed after access. Thus, the initial data is necessarily restored to the memory.
“A 0.24-um 2.0-V 1T1MTJ 16-kb nonvolatile magnetoresistance RAM with self-reference sensing scheme” disclosed by G. Jeong et.al. in IEEE J. Solid-State Circuits 38 (11), 1906 (2003) and “A 512 Kb Cross-Point Cell MRAM” disclosed by N. Sakimura et.al. in ISSCC Dig. 278 (2003) also disclose data access circuit by way of self reference. However, the data access time is very slow (microsecond level).